A loss of joint flexibility is experienced by individuals recovering from neuromuscular diseases, traumatic injuries such as bone fractures, tendon and ligament tears, joint replacements and burns. In order to regain joint flexibility, it is necessary to flex or extend the joint in a repeated, controlled and quantifiable manner. It is also sometimes necessary to apply a relatively small force of a long duration or repeatedly to resolve immobilization stiffness and established contractures regardless of etiology.
Devices have been developed for either flexing or extending joints in a single plane. Examples of these devices are in U.S. Pat. Nos. 4,508,111, 4,397,308, 4,485,808 and 4,538,600, all by Hepburn. These devices generally comprise upper and lower struts which attach to the limbs of the desired joint using an appropriate attachment means such as VELCRO or strapping. The upper and lower struts are pivotally attached to one another at the ends adjacent the joint. The pivotal attachment includes a cylindrical housing with a cam, wherein one of the struts is attached to the cam and the other bears on the cam surface through a bearing spring. Flexing or extending the joint causes a corresponding approximation or alignment of the struts relative to one another and a compression or expansion of the spring. The use of the spring allows a somewhat quantifiable and adjustable constant force to be applied to urge the flexing or extending of the joint.
The devices described in the patents named above are a great advance in that they apply a flexing or extending force on the joint rather than simply immobilizing the joint, but they have several drawbacks. One drawback is that they do not provide for cycled flexing and extending. Recently, it has been found that cycled motion is more therapeutic than static force for treating total joint replacements and in many other therapies. Another drawback is that they pivot at a single fixed axis and move through a single plane. In contrast, the normal motion of most body joints includes pivoting at an axis that slides in relation to the joint to produce a "component motion" and that moves through at least three planes in a "triplanar motion." For example, the human knee joint does not pivot at a single axis. Instead, it pivots at an axis that slides around the femur, so that the lower leg actually moves away from the upper leg as the knee bends. A similar situation exists in the elbow, ankle and many other joints. The failure to accommodate this movement causes a binding of the pivot mechanism of the device, destructive pressure on the internal body joint-bearing surfaces, and migration and misalignment of the device. Accommodating this movement is particularly difficult because, not only is it complex, it also varies greatly from patient to patient.
Other devices exist which do accommodate component motion to allow normal joint response, but these devices are merely braces to limit the range of joint motion. An example of such a device is in U.S. Pat. No. 4,489,718 by Martin. This device may support the knee joint effectively and allow for limited knee motion, but it does not apply any flexing or extending force to rehabilitate the knee and increase flexibility.
The ankle joint is especially challenging to the physical therapist because of the frequency of injury and the complexity of movement. The anterior tibiotalar ligament and other parts of the deltoid ligament are particularly prone to spraining or tearing, typically caused by the foot being forced into external rotation and eversion with respect to the leg. Once the initial trauma has subsided and the patient is able to mobilize the joint, gradual and repeated stretching and stressing of the injured area may be indicated to restore strength and mobility.
Regarding the complexity of movement of the ankle joint, it is well known that movement of the foot with respect to the ankle involves much more than a simple pivoting of the foot about an axis at the junction between the foot an lower leg. Instead, a large variety of movements and combinations of movements are essential. Eversion and inversion refer to movement of the foot about a horizontal axis on the sagittal plane. Abduction and adduction are the movement of the forefoot about a vertical axis. Internal and external rotation are the movement between the leg and hindfoot occurring about a vertical axis. Plantarflexion and dorsiflexion are movement about a horizontal axis lying in the frontal plane which results in vertical movement of the calcaneus. Pronation and supination are functional movements occurring around the obliquely situated subtalar or transverse tarsal joint axis. Because these axes are inclined backward, downward and laterally, pronation and supination also necessarily cause some abduction and adduction, inversion and eversion, and plantarflexion and dorsiflexion. Each of these movements may involve various movements of joint elements with respect to one another, including swing, rotation, spin, roll, slide, and distraction and compression.
Given the complexities of ankle joint movement, it can be seen that attempting to mobilize the joint with a simple pivoting about a horizontal axis will not be effective. In fact, it may be counterproductive because it may cause unnatural straining, compression and distraction of joint elements by preventing the necessary ancillary component movements.